1. Field of the Invention
This invention relates generally to a feedback control system for precise positioning of pneumatic actuators and, more specifically, to a pneumatic positioner used in conjunction with a pneumatic actuator providing precise positioning of the pneumatic actuator by adjusting the flow of air to the pneumatic actuator and having an adjustable gain mechanism for adjusting the sensitivity of the positioner.
2. Background of the Invention
Valtek International, the assignee of the present invention, is a manufacturer of automatic control valves, actuators, intelligent systems, and associated equipment to the chemical, petrochemical, power, pulp and paper, petroleum, and associated process industries. Facilities of these industries utilize control valves to regulate the flow, pressure, or temperature of liquids and gases in various process systems. Control valves are a special type of valve having a power positioning actuator which is responsive to externally supplied signals for operating (i.e., moving) a throttling or closure mechanism located in the valve body. Typical valves include rotary valves (e.g., ball, plug, and eccentric disk) and linear valves (e.g., globe and gate). An exemplary globe-type control valve includes a valve body having an internal passage formed therein with an inlet opening for receiving fluid, an outlet opening for discharging fluid, and a central opening located in the valve body between the inlet opening and outlet opening and forming a valve seat therein. A valve stem, with a valve plug located on one end thereof, is disposed to extend into the valve body and is movable to selectively move the plug onto and off from the valve seat to thereby close the central opening and stop the flow of fluid, or unclose the central opening and allow the flow of fluid, respectively. The other end of the valve stem, opposite that on which the plug is located, is coupled to an actuator typically mounted on top of the valve body. The actuator includes a cylinder, and a moveable piston disposed in the cylinder and coupled to the other end of the valve stem (or could include a diaphragm for operating the stem. A pressurized source of air is supplied to a feedback control system (commonly referred to as a positioner and oftentimes located at the side of the actuator) to direct pressurized air to the cylinder both above and below the piston, in response to control signals, to thereby cause the piston to move to selected positions in the cylinder and thus the plug to move to open the valve a desired amount. By controlling the position of the plug in the valve body, upstream pressure, downstream pressure, and temperature of the fluid flowing through the valve (as well as external variables such as pressure or volume of fluids in tanks connected to the system, pH of fluid in the system, etc.) can be controlled.
While the concept of using the flow of air into a cylinder to position a piston therein may be a simple concept, the regulation of the air flow to achieve precise positioning of the piston within the cylinder requires such a feedback control system. A pneumatic positioner is employed to ensure that the piston is at a desired position within the cylinder and that the piston maintains this position until a change in the position of the valve stem is desired. Thus, the pneumatic positioner provides the appropriate flow of air into the actuator and regulates the air and pressure based on the desired position of the piston and feedback from the piston to the pneumatic positioner.
Feedback of the piston to the pneumatic positioner is generally accomplished by providing a feedback device (e.g., a spring) having one end in communication with the piston (e.g., through a linkage mechanism) and the other end in communication with the pneumatic positioner (e.g., attached to a diaphragm assembly or input capsule of the pneumatic positioner). Tension of the feedback device provides feedback to the positioner, which will vary as the valve stem position changes. Thus, when a feedback spring is employed, the spring-loaded force is applied through feedback linkage and a cam assembly to the input capsule. An instrument signal (i.e. air pressure) is applied between diaphragms of the input capsule which forces the input capsule in a direction that stretches the feedback spring. Movement of the input capsule away from the feedback spring also causes the pneumatic positioner to increase the flow of air into the actuator in order to retract the piston and thus the valve stem into the cylinder. Movement of the valve stem in turn cause the linkage between the valve stem and the feedback spring to apply force to the feedback spring and thus stretch the spring away from the input capsule. When the opposing forces between the input capsule and the feedback spring balance exactly, the system will be in equilibrium and the valve stem will be in the position called for by the instrument signal. If the opposing forces are not in balance, the input capsule will move up or down and the positioner will change the output pressures of the air supply lines, moving the valve stem until the tension on the feedback spring opposes exactly the instrument signal pressure. Thus, air will continue to be supplied into the actuator until the piston and valve stem are retracted a sufficient amount to create a balance between input capsule and the feedback spring.
In order to adjust the flow of air into the actuator, displacement of the input capsule moves an elongate member (hereinafter referred to as the flapper) connected on one end to the input capsule and attached at the other end to another diaphragm assembly (hereinafter referred to as the pilot valve capsule). Displacement of the flapper moves the flapper away from a detection nozzle, which in turn causes movement in the pilot valve capsule. Movement of the pilot valve capsule opens one or more valves and allows a flow of air into the cylinder of the actuator in order to move the piston housed therein. In the prior art positioner, the flapper is secured to the input capsule with an attachment spring. The attachment spring is secured relative to the input capsule and flapper at two positions corresponding to two gain settings, low and high. The low gain setting is achieved by attaching the flapper to the input capsule at the point farther from the detection nozzle. Typically, a low gain setting (i.e., 300 to 1) is a standard setting for a size 25 linear actuator and a high gain setting (i.e., 550 to 1) is standard for a size 50 linear actuator. In order to change the gain setting, the pneumatic positioner must be partially disassembled in order to remove and relocate the attachment spring and subsequently reassembled with the attachment spring secured between the flapper and input capsule at the gain setting. In addition, the gain can only be set at the high gain setting or the low gain setting with no intermediate settings possible.
Thus, it would be advantageous to provide a pneumatic positioner in which the gain can be adjusted without requiring disassembly of the pneumatic positioner. In addition, it would be advantageous to provide a pneumatic positioner in which the gain can be adjusted over a range of gain settings from a low gain setting to a high gain setting.